Gyro-stabilized aerial camera mount



'7' Sheets-Sheet l C. ASCHENBRENNER ET AL GYRO-STABILIZED AERIAL CAMERA MOUNT Sept. 26, v195:30

Filed March 8, 1948 w m m. .N

Sem. Z6, 1950 c. lfxscl-nrqmiiuNr-:R ET A1. 2,523,267

GYRo-STABILIZED AERIAL CAMERA MOUNT im Emmi Filed March Sr 1948 Se 26, 1950 c. AscHENBRENNx-:R ET A1. 2,523,267

GYRo-STABILIZED AERIAL CAMERA MOUNT 7 Sheets-Sheet 3 Filed March 8, 1948 p u n Spa. 1:95@

C. ASCHENBRENNER ET AL GYRO-STABILIZED AERIAL CAMERA MOUNT 7 Sheets-Sheet 4 Filed March 8, 1948 Sept. 26, 1950 c. ,l-'asczr-lL=.:1\1BREI\INEI-1:` ETAL 2,523,267

GYRWSTABILIZED AERIAL CAMERA MOUNT Filed Marcil 8, 1948 7 SheeCS-Sheer. 5

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@rm/@NEM Sept. Z6, 1950 c. AscHENBRl-:NNER ET A1. 2,523,267

GYRO-STABILIZED AERIAL CAMERA MOUNT '7 Sheets-Sheet 6 Filed March 8, 1948 Sept. 26, 1950 c. ASCHENBRENNER ET AL. 2,523,267

GYRo-STABILIZED AERIAL CAMERA MOUNT Filed March 8, 1948 -wwmll \i IIVIIIHIIIII il l In Patented Sept. 26, 1950 GYRO STABILIZED AERIAL CAMERA MOUNT `Claus Aschenbrenner, Malden, Mass., and Ulrich K. Heidelauf and Herman R. Mestwerdt, Dayton, and Bruno K. Wernicke, Trotwood, Ohio Application March 8, 1948, Serial-No. 13,728

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370'0. G. 757) 14 Claims.

The invention described in the following specication and claims may be manufactured and used by or for the Government for governmental purposes, without the payment to us of any royalty thereon.

This invention relates to stabilized supporting means and more particularly to gyroscope stabilized camera supporting and leveling apparatus, primarily employed in the stabilizing and leveling ofvaerial mapping cameras and other apparatus onfaircraft while in flight with respect to a vertical reference.

An object of the present invention is the provision ofextremely sensitive, light and compact gyroscopically controlled, electronically lactuated leveling means for platforms or similar supports on which aerial mapping cameras and similar precision apparatus are mounted.

A further object is the provision of an electrically controlled platform leveling apparatus employing a gravity erected gyroscope for a vertical reference member in which no torque is applied to the vertical reference member or gyroscope during the leveling action to disturb the position of the vertical reference member or precess the gyroscope incident to the leveling action.

A still further object is the provision of apparatus for stabilizing aerial mapping camera-supporting platforms, under the control of a gravity erected gyroscope vertical reference member, in which rotary leveling means are provided for leveling the platform in transverse intersecting vertical planes, the rotary leveling means having minimum inertia, and including dampening control means actuated by the rotary leveling means for exerting a dampening control action on the rotary leveling means in a predetermined ratio to the rate of rotation of the rotary leveling means.

A furtherobject of the present invention is the provision of a support having a camera leveling -mount or platform thereon with an electrically operable means for leveling the platform, including photocell and light-beam projecting means on the platform having vertical reference means for displacing a light beam from the lightbeam projection means incident to relative tilting displacementbetween the platform and the verticalreference means, in which minimum displacements between the platform and the vertical reference means produces maximum displacements of the light beam relative to the photocell means.

.A still further object is the provision of 'an improved photocell and a gyroscope having a vertig cal reference axis for determining the level position of an aerial camera mount, including light beam projection means on the mount and light beam reflecting means on the camera mount maintained in a plane normal to the vertical reference axis of the gyroscope, in which flat wide beams of light are projected onto the reflecting means in transverse intersecting substantially vertical planes and reiiectedk onto photocell devices which are disposed on the mounts in said intersecting planes at opposite sides of the median planes of the fiat light beams, together with light concentrating means located at each side of the median planes of the light beams, when the platform is level and the vertical reference is vertical, for concentrating the light received by each light concentrating means on the respective photocell devices at the opposite sides of the median plane of the light beam, and including :reversible motor driven leveling means for leveling the platform having electronically amplifiedcontrol circuits connected between said photocell devices and the motor driven leveling means for selectively energizing the motor in one direction or the other in predetermined ratio to the light received by each of the photocell devices, together with electrical current generating. means driven by the rotary leveling means for electronically controlling the rate of rotating of the rotary leveling means in a predetermined ratio to the rate and direction of rotation of rotary leveling'means to produce a decelerating or dampening action on the rotary leveling means as the current supplied thereto under control of the photocell devices is reduced.

A still further object is the provision of a leveling platform apparatus for aerial mapping cameras and --precision apparatus carried by the platform in which a vertical reference member on the platform and electrical power operated leveling means for leveling the platform are provided, the leveling means being responsive to angular displacements between the platform and the vertical yreference member to level the platform with respect to the vertical referencemember, including electronically controlled damspening means for damping the leveling action of the platform in a predetermined ratio to the-rate of operation of the electrical power yoperated leveling means.

A still further object is the provision of an aerial mapping camera supporting platform in which improved leveling adjustment means are yprovided between the leveling platform and the camera mounted thereon whereby the camera or 3 other apparatus carried by the platform may be easily, conveniently, and positively adjusted in transverse intersecting vertical :planes relative to the position of the leveling platform, and adjusted in azimuth independently of the azimuth adjustment of the platform.

A further object of the invention is the provision of light beam and photocell electronically controlled leveling control means employing two inclined flat wide rectangular beams of light, projected in transverse intersecting vertical planes, with a tiltable mirror disposed horizontally in the path of said light beams for reflecting and displacing each of the light beams in the transverse intersecting vertical planes in a predetermined direction and ratio to the relative direction and ratio of relative displacement between the horizontal mirror and the camera leveling support in the aforesaid vertical intersecting planes, including two `pairs of side-by-side elongated parallel positive lens elements, preferably planoconveX cylindric lenses each pair being disposed with their adjacent longitudinal edges in one of the median planes of the reflected light beams when the support is level and the mirror is horizontal, together with a pair of photoelectric cell elements for each light beam, located in the aforesaid vertical intersecting planes, each pair of photoelectric cell elements having electronically amplified operating circuits connected to the photocell devices in that plane to be selectively energized in a predetermined ratio to the displacement of the reflected light beam in that plane.

A further object is the provision of improved camera supporting and leveling means for aerial camera supporting platforms, having electronic control means for adjusting the platform, in pitch and roll planes of an aircraft carrying the camera, to a level position relative to a gravity vertical reference, including a pair of reversible eddy current leveling motors having a minimum degree of inertia and operatively connected to the supporting platform to tilt the @platform relatively in the pitch and roll planes, together with light beam photocell control means for selectively controlling the direction and rate of operation of the eddy current motors in a predetermined ratio to the direction and degree of displacement of the platform from a level position, together with a reversible D. C. generator driven by each of the eddy current motors, for introducing an opposing or dampening electrical control potential in the photocell control means in proportion to the rate and direction of rotation of the eddy current motor means, to eifect an electrical damping control action on the eddy current motors in predetermined ratio to the rate of operation of the eddy current motors to decelerate the speed of rotation of the eddy current motor means as the photocell controlled operating potential to the eddy current motors is reduced.

A still further object is the provision of an improved follow-up system for sensitive and follow-up elements carried on a support in an aircraft and sensitive to pitch, roll and azimuth movements of the aircraft, in which said sensitive element comprising a gyro vertical reference member on the follow-up element, the follow-up element being adjustable in pitch, roll and in azimuth, and comprising a gimb-alled supporting platform` for an aerial mapping camera or similar precision apparatus, the platform having photoelectric cell and light beam electronic control means thereon for leveling the platform rela-` tive to the position of the gyro vertical reference member carried by the platform and including electrical dampening control means operable incident to the rate of leveling action of the platform to dampen the leveling action in a predetermined decreasing ratio, as the rate of leveling vaction decreases, to prevent the platform and the platform 1leveling control means from hunting or oscillating as the platform is maintained in level position.

Other and further objects and advantages of the invention will become more apparent from the following description taken in connection with the accompanying drawings in which like reference characters refer to like parts in the several figures.

Drawings Fig. 1 is a top plan view of an improved camera leveling mount, illustrating the same positioned above an opening in the lower portion of the fuselage of an airplane, the camera being omitted.

Fig. 2 is a side elevation of the main frame of the leveling apparatus showing the same mounted on supporting brackets which are secured to the aircraft, and illustrating the cushioned securing means between the supporting bars which are secured to the aircraft, and the main frame of the leveling apparatus, the gimbal rings and certain parts supported thereby being omitted from this figure of the drawing. Parts are broken away and shown in section.

Figure 3 is an end view of the main frame showing the two gimbal rings in position, parts being broken away and shown in section.

Figures 4 and 5 are vertical sectional views taken through the photoelectric cell and light beam projection unit, the gyroscope, mirror, and pendulous drive motor for the gyroscope rotor being omitted from Figure 5.

Figure 6 is a top plan view of the improved photocell light beam projection control unit, illustrating a portion of the azimuth ring on which the unit is mounted, parts of the casing being broken away and shown in section to disclose the locations of the electronic amplifier tubes which are controlled by the photocells.

Figure 7 is a rear View of the photocell control unit, more clearly illustrating the preliminary leveling adjustment features.

Figures 8 and 9 are detail elevation and bottom plan views of one *of the eddy current leveling motors, certain parts being broken away and shown in section in Figure 8.

Figure 10 is a sectional view through one of the eddy current leveling motors.

Figure 11 is a cross sectional View of the follow up control potentiometer for the azimuth motor follow up control circuit.

Figure 12 is a detail view of the azimuth motor arrangement.

Figure 13 is a detail view of the supporting platform or ring, diagrammatically illustrating a mapping camera in position thereon, parts being broken away to more clearly illustrate the spirit levels for determining the preliminary adjustment of the camera ring to a level position.

Figure 14 is a schematic wiring diagram illustrating a wiring circuit for the leveling apparatus and disclosing somewhat simplified electrical and electronic control circuits.

Figure 15 is a vertical section more clearly showing how the camera supporting casing or ring is mounted on the azimuth ring and one of s the three leveling jacks for leveling the camera with respect to the azimuth ring. One of the azimuth ring supporting bearings is also shown.

Figure 16 is an enlarged fragmentary vertical sectional view taken on a vertical plane passing through the adjustment means for rotating the azimuth ring, showing a portion of the camera ring or casing and one of the adjustable camera supporting jack screws.

Referring more particularly to Figure 1 of the drawings, the reference numeral I indicates a lower portion of an aircraft fuselage having an opening 2 through the bottom thereof with brackets or supporting shelf members 3 at opposite sides of the opening, on which are mounted and secured by bolts 6, 6 a pair of laterally spaced, 'longitudinally extending supports or bearing arms 5. The opposite ends of each of the bearing arms or supports .5-5 as seen in Fig, 2 are recessed to receive shock and vibration absorbing supports or cushion members l', each member I having a threaded extension which is secured in the bearing arm by a nut The cushion member I includes an intermediate resilient portion I which is surrounded by a sleeve member 9 for limiting the degree of expansion of the resilient portion -'I incident Yto downward pressure applied to the upper end of the cushion member l. The upper ends of the cushion members are reduced and vthreadedsto form stems which are secured in suitable openings Ill formed in countersunk recesses I I in a somewhat rectangular shaped main or supporting frame l2. The frame l2 resting on the resilient cushions in parallel relation to the bearing arms 5, is secured to the supporting bearing arms through the cushion members 'I by the clamping nuts I3-I3.

Depending bracket arms or hanger members I'4-I4 are secured to the inner portion of the supporting frame I2 in diametrically opposite relation, the lower ends of these hangers being provided-with inwardly extending trunnion members I5-I5. An outer gimbal frame I6, as seen in Fig. l, is journalled at its opposite sides on the trunnion members I5I5 in suitable antifriction bearings I'I-I l, shown in detail in Fig. 3, permitting tilting displacement of the outer gimbal frame I6 about an axis transverse to the longitudinalaxis'of the rectangular frame I2, the tilt laxis ofthe outer gimbal frame being preferably disposedperpendicular to the longitudinal axis ofthe aircraft carrying the leveling device. The outer gimbal frame le is somewhat D shaped in form, as best seen in Fig. 1, having an end portion Iaextending parallel to the end portion I2df of the main supporting frame I2.

A two-phase eddy current motor, indicated generally at I8 iii-Figure l, and Shown in detail in Figs. 8 to `l() and later described in detail, is rigidly secured by a bracket I5 fastened at I8 to the gimbal frame I. The motor IS, through a reduction gear train I9, drives a Worm or screw shaft 2II on which is threadably secured a travel-- ingnut 2l. The nut 2l is slidably received in a yoke member 22, journalled in suitable bearings 23, located in the rectangular frame I2, as seen in Figs. 1 and 8. Rotation of the screw shaft 20 by eddy current motor I8 causes the gimbal frame I6 carrying the eddy current motor I8 to be tilted about the axis cf the trunnion members I5, I5.

J ournalled withinthe outer gimbal frame I6 on ka tilt axis transverse to the tilt axis of the outer gimbal frame is an inner gimbal .frame 24 with supporting lbea-rings at the opposite sides, indicated a-t 25. This innerk gimbal frame24 as best seen in Figure l5 constitutes a camera supporting platform or sensitive supporting element on which an aerial mapping camera C 0r other apparatus adapted yto be mounted. The inner gimbal ring or camera supporting platform 24 carries an azimuth mounting ring l26, rotatably journalled thereon for movements in azimuth on suitable antifriction bearings 26. Separate electric motor control means are provided, later to be described, for adjusting the position of the mounting ring 25 in azimuth.

A second, two-phase eddy current leveling motor 21 is mounted on the outer gimbal frame YI6 as seen in Fig. 1, and is similar in construction to the first mentioned eddy current motor I8. The eddy current motor 21, through a suitable gear reductiony train, drives a worm or screw shaft 21a with a traveling nut thereon similar to the nut v2I, slidably engaging .a shifter `yoke member 28 that is journalled in suitable bearings: on the inner gimbal frame or supporting `platform 24. Rotation of the motor 2I actuatesrthe nut to tilt the inner gimbal frame relative'to the outer gimbal frame I6 on its bearings 25 about ,y an axis perpendicular to the tilt axis of the youter gimbal frame I6.

The azimuth mounting ring or platform support 26 is provided on its upper surface with two sets of threaded holes 29-29 and 29a. Adjustable seats or sliding supporting plates l30 and 330, are provided for supporting a Vmapping camera or other conventional precision apparatus on the azimuth mounting ring with its iil'm axis in accurate longitudinal alignment with the longitudinal axis of the rectangular frame I2 `and constitute seating -means for the-adjusting means for adjusting the level of a camera C or apparatus to be carried by the'ringZG. -The holes 29-29 and surrounding plates -30-30 are normally disposed in a plane passing through the tilt axis of the inner gimbal ring or frame 24, .while each of the other holes 29e-29a and adjustable plates Sila--Sa are equally spaced, circumferentially, around the ring '25 in the same direction e. from each of the holes 29 and plates -39 for reasons later to be described.

The mapping camera C is mounted on anannular ring or casing member 3l of substantially the same diameter as the inner openingin the supporting or azimuth ring 26. This mounting ring 3l is shown in Fig. 13 and is provided with two diametrically opposite supporting brackets Sla and 3Ib having threaded holes 32, each receiving a supporting jack screw member 34. Each jack member 34 as best seen in Figure '15 is formed with a spherical bearing head 34a at its lower end adapted to seat in one or theother of the spherically socketed slidably adjustable plates 30 or 30a which are retained on the azimuth ring 26 in diametrically opposite relation. This arrangement positions the camera with the longitudinal axis or movement of the lm al- Ways fixed relative to the position of the azimuth ringr 26, since either of the supporting jack screws 34 may be adjusted independently, the camera C can be conveniently levelled or adjusted to dispose the focal plane parallel to the plane of the inner gimbal ring 24 or azimuth ring 26, and the longitudinal iilm axis in a plane parallel to the longitudinal axis of the main frame l2. Tightening of securing bolts 25. secures the camera supporting brackets 3 Ia., 3 Ib in a fore and aft leveladjustment with respect to the azimuth ring 2r?. The supporting jack screw `34h seats in one or the other of the recessed plates asaaee'? f3`0a,permitting the camera casing or ring 3i to be secured on the azimuth ring 26 with the direction of film movement in either of two directions which are 180 apart. Adjustment of the screw 34D and tightening of the securing bolt 35 permits the camera casing ring 3| and a camera thereon to be leveled and secured in a transverse plane, thus permitting the axis of the lens L of the camera to be adjusted to a vertical position fin two transverse intersecting planes relative to the azimuth ring 26 when the azimuth ring is Tin stabilized position. The bolts 35 pass through the hollow jack screws 34 and 34h and are provided with slotted heads which engage the upper 'ends of the jack screws. The ends of the bolts 35 extend through central apertures in the plates 130 and 38a so that tightening of the bolts 35 sercurely seats the spherical ends 34a of the jack '.members in the spherical depressions of the plates 30-3Da. The camera mounting ring 3| is provided with camera supporting brackets 3la, 3lb secured 0n the exterior of the ring 26 in '.diametrically opposite relation by fastenings 31.

The azimuth camera mounting platform or .'azimuth ring 26 is adjusted in azimuth in the :inner gimbal ring 24 to position the camera so ithat the pictures taken thereby will be oriented iin the same direction as the ground track of be aircraft regardless of cross winds and drift (conditions. Projecting from the lower edge of Jthe inner gimbal ring 24 is a supporting bracket or plate extension 38 on which the adjusting mechanism for the azimuth ring 26 is mounted. This mechanism is best disclosed in Figure 16 and is enclosed in a casing 39, secured on the bracket 38, the mechanism comprising a worm shaft 40 journalled in the casing 39 and driven by a reversible D. C. motor 4I through a suitable coupling member, the Worm element on the worm shaft 40 being disposed in meshing relation with a worm segment 42 projecting from a peripheral flange on the azimuth ring 26. Rotation of the motor 4| in one direction or the other rotatably .adjusts the azimuth ring 26 within the inner gimbal member 24.

Secured in one end of the casing 39 is a follow- 'up control device for the azimuth motor 4! as iindicated at 43. rEhe follow-up control device is (driven by the worm shaft 48 through a reduction (gear train 48a and includes apotentiometer which iis electrically connected to rebalance the azimuth :motor control circuit, later described, as the azi- .'muth ring 26 is rotated. A previous unbalancing (of a bridge control circuit to the motor 4i by the :manipulation of a manual operable azimuth con- '.'trol potentiometer initiates the azimuth adjust- :ment

Mounted on the azimuth ring or platform 26 :'is a photoelectric cell and light beam control unit :indicated generally at 44 and best seen in Figs. -4 to 6. The control unit 44 is carried by an adjustable two part bracket 45, rigidly secured to the azimuth ring by screw iastenings 46 passing through the bracket iiange 41 and threaded in suitable openings in the azimuth ring. The bracket 45 includes an upper adjustable portion :'48 horizontally pivoted at 49 to a lower bracket portion 55, a set screw l being provided in the v.upper bracket portion for adjusting the position of the upper portion 48 in 9, vertical plane, and the `photocell and light beam projection unit 44 rcaritied thereby. A bolt 52 is provided to draw #the set screw member 5i into securing relation against the lower bracket portion 50, rigidly se- 8 curing the upper and lower portions of the bracket in their adjusted positions.

The photoelectric cell and light beam projection control unit 44 is also rotatably adjustable about a pivot or dowel pin 53 in a plane transe Verse to the axis of the horizontal pivot connection 49. Securing bolts or screws 54 as best seen in Fig. 1 extend through arcuate slots 55 in the upper bracket portion 48, the radius of curvature of the slots having a common center located at the axis of the dcwel pin 53. The bearing plate or face 56 0f the photocell unit 44 is secured against a face plate 51 formed on the upper portion 48 of the bracket 45. The portion of the bearing plate 56 which is located above the bracket member 45 is provided with spaced integral lugs 58, having axially aligned adjusting screws 59 threaded therein and extending toward each other into engagement with the opposite faces of a lug 60 that projects upwardly from the upper edge of the bracket 45. When the two clamping screws 54 are loosened, the two adjusting screws 59 may be rotated to positively adjust the photocell unit 44 in a plane normal to the axis of the dowel pin 53 and perpendicular to the plane of adjustment of the upper bracket portion 48 about the axis of the pivotal connection 49.

The photoelectric cell and light beam projection leveling control unit 44, as seen in Figs. 4 to 6, includes a gyroscope and two sets of photocell and light beam control elements. Secured to the bearing plate 56 of the control unit 44 is a bracket member 6i, on which is mounted a vertical reference member, arranged for tilting displacement in transverse intersecting planes, the vertical reference member comprising a balanced gyroscope rotor 62 with means for erecting the spin axis of the rotor to a true vertical reference axis, the erecting and supporting means for the rotor 62 being a gravity operated vertical 0r pendulous member 63 suspended from the gimbals 64. The gravity operated vertical reference member 63 includes an electric motor having a drive shaft 63 which terminates in a ball shaped head at the gimbal tilt axis and the gyroscope rotor 62 supported on this head, an inverted V- shaped opening being formed in the central portion of the underside of the rotor engaging the bail-shaped head on the end of the motor drive shaft and constituting a irictional driving means between the motor drive shaft 63 and the gyroscope rotor 63 and also constituting frictional erecting means for the spin axis of the gyroscope rotor 62. Centrifugally releasable drive means are provided between the rotor 62 and the motor drive shaft for initially providing a high driving torque relation between the motor shaft 63' and the rotor 62 until the rotor is accelerated substantially to the maximum speed, after which the centrifugal clutch automatically is released, permitting the rotor to be driven solely by frictional Contact between the ball element on the endoi the motor shaft and the sides of the inverted V-shaped supporting recess in the under side of the rotor. When the rotor 62 tilts relative to the axis of the motor drive shaft an Iunequal Iapplication of the motor driving torque is frictionally applied to the sides of the V-shaped opening in the rotor, producing a corresponding precession and erection of the spin axis of the rotor 62 to the vertical reference axis of the motor drive shaft 63. The exterior surface of the rotor 62 is tapered upward as shown in the drawings, its top surface being provided with a circular optical afs-cages flat and light reecting surface in the form of a mirror 65, secured in a plane perpendicular to the rotor spin axis.

The upper portion of the bearing plate 56 of the controll unit 44 is somewhat dome shaped, having a substantially truncated, four-sided, pyramidal form with an opening 6B through each of the four inclined walls, the openings 66 being in two transverse vertical planes intersecting each other at the tilt axis of the rotor, one of these planes extending through the spin axis of the rotor at its tilt axis and through the center of the azimuth ring 26, in other words in the plane of pitch of the aircraft carrying the device. The other intersecting plane passes through the other two openings S6 and is, of course, perpendicular to the first mentioned plane, intersecting same at the tilt axis of the rotor 62, and is in the roll plane or axis of the aircraft.

A light beam projection device 61 and Gla is secured in one of the openings 96 in each of the aforesaid intersecting planes, having a light projection axis GE-X which inclines downwardly toward the center of the mirror 65, when the mirror is level and the azimuth ring 26 is level with the photographic axis L of the camera C in a vertical position. Each light beam projector includes means for projecting a beam of light in a substantially flat wide and rectangular form, onto the surface of the mirror at an angle of incidence of about forty-five degrees, the beams of light striking the mirror surface approximately at the spin axis of the rotor in the aforesaid intersecting planes. In each of the aforesaid intersecting planes is a pair of photoelectrie cell devices, indicated generally at E8, the photocell devices each comprisingT a pair of photoelectric cells 69 and li?, and 69a and 70a, disposed in spaced parallel side-by-side relation at opposite sides of the median plane of each of the reflected light beams, when the mirror 65 is level and the supporting platform or azimuth ring 26 is level. Each pair of the photoelectric cell devices are connected to leveling control circuits, later to be set forth, for controlling the rate and direction of rotation of the eddy current leveling motors I8 or 2l in order to correct the pitch and roll movements of the main frame I2 and maintain the camera in level position. A small casing 3| surrounds each pair of photocell elements 59 and 19, and 53d and lila, with a balile plate I2 disposed between each pair of photoelectric cell elements.

Elongated, positive, or plano-convex-cylindric lens elements l3-T3 and i3d- 13a are disposed, one in front of each of the photoelectric cells |59 and 'Hl and G9a and "Isa, the spaced relation of the lens elements from the cathodes of the photoelectric cells 69 and 'I9 (99a and 19a) being substantially equal to the focal length of the cylindric lens elements. These lens elements are secured in the hood portion of the unit 44 by supporting arms lli which extend through the openings 66. The adjacent edges of each pair of lens elements I3 lie in juxtaposed relation to the median plane of the projected light beam 93X when the supporting platform 29 and the reflecting surface of the vertical reference member 62 are inv predetermined relative level reference positions.

The cathodes of the photoelectric cells 69 and 'i0 associated with each of the cylindric lens elements 'I3- 73 and of the photocells 69a and 10a associated with lens elements i3d-'l3nt are elongated, each disposed substantially at the focus of the cylindric lens element, the elongated concave cathode element being provided in each photocell element behind an equally elongated anode, so that reflected light beams from the mirror 65 passing through either of the cylindric lens elements will be concentrated onto the cathode elements of the respective photocells.

The two projected and reflected light beams are flat, rectangular, comparatively wide and thin, having a median plane which normally passes through the juxtaposed edge portions of each pair of the side-by-sidecylindric lenses 13, 13, and therefore When the wide and relative thin beam of light is concentrated on each of the photocell cathodes a minimum displacement of the light beam transverse to its median plane, due to slight relative tilt of the gyroscope reflecting mirror 65, causes a maximum change in the ratio of light concentration on the two photoelectric cells 69 and l0 or 59a and 19a in that tilt plane to occur. A suitable slit shaped aperture or reticule plate is provided between the lens elements and the light source of the projection devi-ce 51, this plate being provided with a narrow elongated rectangular transparent slit. The lens of the projection device 61 produces a sharp image of this slit in the plane of the dividing cylindric lenses 73, 13.

Located within the photocell light beam unit 44, and connected to each of the photocells 69, 10, 69a, ma, is a dual cathode, plate and grid (three element) vacuum tube 99', |99, 99a, lila, each plate thereof being connected to one end of the primaries |03, |94 or Htl, Il! of one of the phasing transformers |05, H2, or Id, IIZa, disclosed more clearly in the simplified wiring diagram illustrated in Fig. le. The plates |92, |02 and |99, |99 of the tubes 99 and IBB are each connected to one end of one of the reversely wound primaries |03, |04 or II, III of a pair of phasing transformers |05, I I2 while the grid elements 91, 98 of the tubes 99, |06 are connected to the curved cathodes of the photoelectric cells 69 and 19.

The outputs of the secondaries E05 and |I3 of the two phasing transformers and ||2 associated respectively with the photoelectric cells 69 and "I0, are connected in series in opposition to each other to an intermediate amplifying state, and induce an alternating current potential, varying in amplitude and phase, depending upon the relative degree of illumination of one or the other of the photocells 69, 10 or 69a and 70a, incident to the amount of deflection of the light beam onto one or the other of the adjacent cylindric lens elements. This electrical output controls the rate and direction of rotation of one of the eddy current leveling motors 2l or I8 depending upon which photoelectric cells are excited to the greater extent by a change in deflection of its associated light beam.

Referring to Figs. 1, 8 and 9, the respective eddy current motors I8 and 21 each drive one of the D. C. generators I8a and Z'Ic through suitable reduction gearing |91). The electrical output from each of the D. C. generators lila and 21e is connected to the grid and cathode elements of at least one of the vacuum tubes 99-I9B (or BSc-mila), thereby impressing a variable positive or negative current potential on the grid of that vacuum tube, depending upon the rate and direction of rotation of the respective eddy current motors. This produces an electrical dampening control effect on the eddy current motor circuits for quickly decelerating the speed of the eddy current motors when a reduction in the current supplied to the eddy current motor under the control of the photoelectric cell devices takes place. As the eddy current motor decelerates, the electrical output from the D. C. generator correspondingly decreases and the dampening control effect is correspondingly reduced. The eddy current motors I8 and 21, and the D. C. generators, |80, and 2`|c driven thereby, as well as the drive gearings between these motors and the generators, are similar. It is essential that the inertia of the rotating parts of the eddy current motors be as low as possible. The reference numeral '|6 in Fig. 10 indicates the casing of one of the two phase A. C. eddy current motors, the motor having a drive shaft 16a on which is secured an armature 1l, in the form of a thin aluminum cup. This armature is rotatable between the inner and outer laminated cores T8, 18', the field windings thereof being disposed in a ninety degree outof-phase relation, one of these elds being supplied with an A. C. potential from the same A. C. power source or transformer that supplies the A. C. potential to the phasing transformers IUE-I I2 previously referred to, while the other field of each eddy current motor is supplied with an A. C. potential varying in magnitude and phase as the degree of conductivity of one or the other, or both of the two photoelectric cells is changed.

Limit switches 8I-8I and Sla-Sla, are provided respectively between the main frame I2 and the outer gimbal frame I6, and between the outer and inner gimbal frames I6 and 24 for respectively limiting the tilting action between the main frame I2 and the camera supporting platform 26 in transverse planes thus limiting the degree of tilting action between the azimuth ring 25 and the main frame I2 in pitch and in roll. The limit switches are all preferably microswitches of the normally open type, the switches 8|a-8Ia being secured to the outer gimbal frame I6 at opposite sides of the inner gimbal frame tilt axis, suitable actuating arms BIb-Blb project from the inner gimbal frame to positions for selectively engaging and closing one or the other of the microswitches incident to a predetermined degree of tilt of the inner gimbal frame relative to the outer gimbal frame. The other microswitches 8|-8I are secured in superimposed relation on the outer gimbal frame I6 between a bifurcated actuating arm 8|c, secured to the main frame I6 at one side of the outer gimbal frame tilt axis. In the arrangement disclosed in the drawings the limit switches 8| and Sla are selectively closed by a predetermined degree of tilt of the ring 26 in pitch and in roll and respectively control the energizing of electrical bucking coils which oppose further continued operation of the respective eddy current leveling motors I8 and 2l. This arrangement is schematically illustrated in the Wiring diagram shown in Fig. 14. In practice four hundred cycle 115 v. A. C. eddy type servomotors are employedy and the A. C. source is 400 cycles at about 115 v.

The camera mount is preferably provided with suitable quick disconnect gang terminals and manual adjustment controls, multistage ampliers, D. C. current supply means, such as conventional inverters, as well as intervalometer controls for the operation of the camera, all mounted in the aircraft carrying the device and connected to the apparatus of the subject invention through the quick "disconnect terminals just referred to.

The quick disconnect gang terminals and the distributor locations are indicated at 82 having multicable conduits 83 leading therefrom to the respective electrical elements carried by the inner and outer gimbal rings 24 and I6, suitable channels being formed in the parts for accommodating these conduits, as well known in con ventional electrical wiring practice. No particular reference is made to the speciiic manner in which these conduits are secured in and to the various elements of the apparatus other than to indicate that they must be flexible where relative movements between the parts take place, such as the relative tilting movements between the gimbal frames IE and 24 and the main frame I2, and to permit relative azimuth adjustment of the azimuth ring 26 and adjustment of the photoelectric cell unit 44 relative to the ring 25 on which it is mounted. The simplified schematic wiring diagram, later referred to, should make these conventional connections clear and understood by any one skilled in the art to which the invention appertains.

Located on the camera casing ring assembly 3| as seen in Fig. 13 are two spirit level indicators 84 and 85. These levels are secured in perpendicular relation to the photographic axis of the camera or lens axis L and perpendicular to each other. When the camera is mounted on the main frame, the levels are preferably disposed in pitch and roll planes of movement of the aircraft.

Since the photoelectric cell control unit 44 is capable of adjustment on the azimuth ring 26 in pitch and roll directions, as above pointed out, and since the camera casing 3| is independently adjustable relative to the azimuth ring 26 in pitch by manipulation oi the two fore and aft supporting jack screws 34, and in roll by the adjustment of the offset jack screw 34h, as before set forth, it should be understood that the azimuth ring or supporting platform 26 can be accurately leveled by adjustment of the photoelectric cell gyroscope control unit 44 thereon, and the camera itself can be independently adjusted by leveling the camera casing 3| through adjustment of the jack screws 34 and 34h While observing the positions of the bubbles in the spirit levels 84 and 85 to thereby dispose the camera axis L in a true Vertical position. This latter adjustment can be made even though the azimuth ring 25 is stabilized in a slightly outof-level position, but it is preferable to adjust the azimuth ring to a true level position and then level the camera on the azimuth ring by means of the spirit levels and jack screws 34 and 34h.

In Fig. 14 a simpled schematic wiring or circuit diagram for the camera leveling apparatus is disclosed. In actual practice the control circuit of a somewhat more complicated type including conventional multistage electronic amplification, smoothing circuits for operating the eddy current leveling motors I8 and 21 and the azimuth motor 4|, also protective fuse control devices and other conventional adaptations are employed, which are all well known to those skilled in the art to which the invention apertains and therefore not specifically included in the circuit diagram. The wiring diagram in Fig. 14 also discloses, for simplicity, the use of batteries as a source of D. C. voltage instead of conven- 13 tional rectified electric power sources which are usually employed.

Each of the eddy current leveling motors I8 and Z'I for effecting leveling movement of the camera supporting platform or azimuth ring in pitch and in roll is controlled by one pair of the photoelectric cell elements 69, 'I0 or 69a and 10a, the electrical control lcircuits-,for effecting these two transverse leveling actions being substantially identical in arrangement and operation; Similar reference characters will therefore be used on both of the photoelectric cell control circuits as disclosed in Fig. 14 except that the exponent a is added to the reference characters which are applied to the second or roll leveling circuit. Only the pitch leveling circuit will be specifically described employing reference characters which omit this exponent a. This description will therefore apply to the second or roll leveling control circuit if it is observed that similar parts are indicated in both pitch and roll circuits by similar reference numbers with the exception that certain detail elements previously referred to by entirely diiu ferent reference characters are referred to in the roll circuit by their original reference characters, such as the reference to the eddy current leveling motors I 8 and 21 and the D. C. generators I8a, and 2"!0 driven thereby. For example, in the pitch or fore and aft leveling control circuit the two side-by-side photoelectric cell elements are indicated at 59 and '|0 for selectively receiving the fiat concentrated light beam from the light source 6l and the eddy current leveling motor is indicated at I8, having a D. C. generator I8a driven thereby while in the second or roll leveling control circuit the two side-by-side photo-- electric cells are indicated at 69a and 10a for selectively receiving the wide reected and concentrated beam of light from the light source 61d through the lens elements (13a- 13a in Fig. 5). The reference numeral 86 in Fig. 14 indicates the primary of a power transformer having a predetermined A. C. voltage impressed thereon., for example, 115 volts, 400 cycles. This transformer is provided with center tap secondaries 81 and Bla, supplying electric energy to the pitch and to the roll photoelectric cell ele.- ments G9 and I0, andy 69a and 10a. The primary 86 also induces electric energy in the secondaries 88, 88a, the center tap secondary 89, and the secondary 90, respectively, supplying current to one phase of each of the eddy current motors |8 and 2T, operating the azimuth motor relays, and for supplying an electrical energy to the azimuth adjustment control circuit. The secondaries 88 and 88a, respectively, are connected to one phase of each of the eddy current leveling control motors I8 and 21 through conductors 9| and 92, and conductors 9|a and 92a, these phases of the eddy current motors being indicated at 93 and 93a.

A pair of conductors 94 and 95 are connected to the opposite ends of the center tap secondary 8l, While the center tap of this secondary is connected to a conductor 96. As before indicated the photoelectric cell units 6.9 and 10 are each provided'with an anode and a curved plate cathode, these cathodes being respectively connected to the grid elements 91 and 93 of the vacuum electronic tubes 99- and |00, the tubes 09and IM being provided with cathodes 9S" and 99, and |00 and |00, connected respectively to the conductor 95 through |0| and |0I. Thepates |02 and |02 of the tube 99 are connected respectively to the adjacent ends ofthe opposing primaries |03 and |04 of the phasing transformer |05, this transformer having a secondary |06 connected at its opposite ends to conductors |01 and |08. The plates |00 and |00 of the tube |00 are respectively connectedtothe remote ends of a second pair of oppositely wound primaries ||0 and IIfI of a second phasing transformer |I2 having a secondary |I3, connected in series with the secondary |06 by the conductor |08; The adjacent ends of the phasing transformer primaries IOJand I I I are connected respectively tothe conductors 9'4 and 95.

From the above it will be clear that when one or the other of the tubes 99 or |00, or tubes 99o or Iaf, is conducting, a one-half wave potential is introduced in each of the primaries |03 and |04 of the transformer |057 or in each of the primary coils ||0 and. of the transformer II2 (or in each of the primary coils I 03a and Ia or I09a and IIOa, respectively of the transformers |051; or |I2a). Since the primaries |03 and |04 of the transformer |05 are connected to the-tube 99 to pass current in one direction only, while the primaries I|0 and III of the transformer I'I 2 are wound and connected through the conductors 94 and 95 to the tube |00 to allow current ow in one direction only, the induced electrical potential in the conductors |01 and |08 (or |01a|08a will be an alternating current, the phase of the current being determined by which one of the tubes 99 or |00 (or 99a, or I00a) is conducting. If both tubes 99 and |00 are equally conductive the current potential induced in the conductors |01 and |08 by the two secondaries I0@ and ||3 will be equal and of opposite phase and the output therefor be Zero. A fixed grid bias source ||4 is provided, including fixed resist-org ||5 and IIS establishing a conventional negative bias on the grids 01 and 98 respectively, of the vacuum tubes 09V and |00.

The D. C. generator |8a, before referred to, whichV is driven by the pitch control leveling motor I8, has its electrical output connected through conductors |I6, and |I6 to the grid and cathode elements of at least one of the two dual tubes and |00, resistor Ill and the resistor IIB being interposed between the connected tubes and the D. C. generator, so as to determine the potential impressed on the grid of the connected tube, depending upon the rate and direction of rotation of the D. C. generator |80, which in turn depends on the rate and direction of rotation of the eddy current leveling motor I8.

TheA conductors |07 and |08 are connected to an intermediate electronic 4amplifying stage, indicated generally at IES, the conductor |01 being connected to the grid element |20 of the vacuum tube While the conductor |08 is connected to the cathode elements |2| and |21, A predetermined negative bias is imposed on the grid |20 through the conductors |08 and |01 by a D. C. source indicated at |22. This intermediate amplifying stage I|9 is conventional, the plate |23 of the vacuum tube H0 being connected to the positive side of a D. C. source such as the battery |24 with a resistor IZES interposed in the circuit, the other side of battery is grounded at I24G. The conductor 96 as illustrated is connectedv to both cathodes I2I and IZ'I of the amplifier tube ||9 the conductor 00 being connected to the negative terminal of the battery |24 through the ground connections 06g; the fixed resistor I25S is interposed betweenthe plate |23` of the tube |.I0 and the positive side of the power source or battery |24.

A simplified form of comparative voltage amplifier is disclosed at |26 including the vacuum tube I |9, the second cathode |21 being connected to ground through connection 96g and through |0| and to the positive side of the battery |28, the negative terminal of the battery |28 being connected through a conductor |29 to the grid |30 of the tube, a xed resistor |29 being interposed in the conductor |29. A conductor |3| connects the plate |23' to one end of a primary |32 of a transformer |32 in the usual manner, the other end of the primary |32 being connected by a conductor |33 to the positive terminal of the battery or D. C. power supply |24. The grid |30 of the vacuum tube ||9, and the plate |23 are connected together by a conductor |34, having a condenser |35 interposed therein, in the conventional manner.

The transformer |32 has a center tap secondary |36-|36 and in addition each end of the secondary |36-|36 is wound with one of a pair of opposing or bucking coils |31 and |38 with suiicient turns to oppose or buck the current potential induced in the associated one of the secondary coils |36' when the coils are energized. One end of each of the bucking coils |31-|38 is connected by conductor |39 to the conductor 92, the other ends of the bucking coils |31 and |38 are connected to the conductor 9| by conductors |40 and 4|, each of the conductors |40 and i4! having one of the normally open tilt limit or mocroswitches 8|-8l therein, these switches are the pitch control limit switches previously referred to for limiting the degree of displacement in pitch of the azimuth ring 26.

The opposite ends of the center tap secondary |36-|36 are connected through conductors |42 and |43 to the grids |44 and |45 of the vacuum tubes |46 and |41, located in a nal amplifying stage |48. A battery or D. C. source |49 is provided, connected at its negative terminal to the secondary of the transformer |32 intermediate the two secondary coils |36-|36. The positive terminal of the bias battery |49 is connected to the cathodes |50, |5| of the tubes |46 and |41 by a conductor |52. The plate circuits for the tubes |46 and |41 include a D. C. power source or battery |53 connected at its negative terminal y to the cathodes |50 and |5| through the conductor |52. The positive terminal of battery |53 is connected to a transformer |55 at the center tap, intermediate the primaries |54-|54. The outer or opposite ends of the primary |54|54 are connected to plates |56, |51 of vacuum tubes |46, |41 by the conductors |58, |59 in the conventional manner.

The ends of the secondary of the transformer |55 are connected to a second phase |6| of the two phase eddy current leveling motor i8 by conductors |62 and |63, this phase being displaced in a ninety degree out-of-phase relation relative to the first phase 93 by a condenser |64 which is interposed in the conductor |62 to shift the phase of the current from the transformer secondary |60 to a 90 out-of-phase relation with respect to the phase of the current from the transformer secondary 88.

A manual azimuth adjustment control potentiometer |66 is provided, having an adjustable Contact or slider |61, which is adjustable between the ends of the potentiometer coil |66 to selectively adjust the azimuth position of the azimuth ring or camera supporting platform 26. The

azimuth control circuit follow up or balancing potentiometer device 43, reference being also made to Figs. ll and l2, comprises an insulating base mounted within the azimuth motor casing 39. A follow up potentiometer coil |68 is secured in an annular channel in the base in the conventional manner, a slider contact |16 being secured to an annular flange |69 formed on a rotatable center sleeve |10 which is suitably journalled in antifriction bearings |1|. The periphery of the flange |69 is provided with gear teeth |12 meshing with a small gear |13 of a twin gear unit having a large gear |14 meshing with a reduction gear train 43a before referred to, driven by a gear xed on the end of the azimuth motor shaft 40 carrying the worm gear before referred to. The twin gear unit |13, |14, is journalled for rotation on a standard |15 secured in the base of the potentiometer casing. Rotation of the azimuth motor 4| adjusts the slider |16 with respect to the ends of the potentiometer coil |68 until the bridge circuit to the azimuth motor polarized relay control devices is again balanced, causing the motor 4| to stop. Conductors |11 and |18 respectively connect the manual and follow-up slider contacts |61 and |16 to the primary |19 of an azimuth control transformer having a secondary 8| Manual adjustment of the azimuth control slider contact |61 from the center or balancing position unbalances an A. C. bridge circuit, causing an alternating current flow through the primary |19 of the azimuth transformer |80, inducing a current in the secondary |8| the mean polarity of the induced A. C. potential in the conductors |82 and |83 connected to the secondary |81, depending upon which direction the bridge circuit is unbalanced. The conductors |82- |83 are connected to a conventional electronic amplier indicated generally at |84, the conductor |82 being connected to the grid |85 of a conventional vacuum tube |84a, while the Conductor |83 is connected to the cathodes |86 and |81 of the tube through a conductor |86. A negative grid bias source |68 is interposed in the connection |83, supplying a negative bias to the grid |85 of the tube. The plate circuit includes a battery, or a conventional inverter type of power source |89, connected by conductors |9| and |93 to the plates |96 and |92, xed resistors |9| and |93' respectively being interposed between the plates and |92 of the tube |84@ and the D. C. power source |89. The manually adjustable azimuth control potentiometer |66 is located at any convenient station within the aircraft and is preferably provided with a calibrated dial or adjustment indicating pointer, indicating the position of the slider |66 in terms of the degree in the azimuth position of the azimuth ring 26 from a central position where the bridge circuit of the potentiometer 43 balances the bridge circuit of the potentiometer |66 with the pointer indicating zero.

As schematically ilustrated in the wiring diagram, in Fig. 14, simplied electronic ampliers are shown, although in actual practice multistage amplification is employed where greater electrical operating potential is required, particularly in the control and operation of the leveling motors I8 and 21 and the operation and control of the azimuth motor 4 I.

In the azimuth control circuit, the negative terminal of a battery or D. C. source |88 is connected to the grid of the amplifier tube |84a by a conductor |98 with a resistor |99 in the connection, supplying a negative bias to the grid |95.

17 The `conductor |94 also connects the grid |95 to the plate |90 and a xed condenser |96 is interposed in the connection between the plate |90 and the grid |95 in the conventional manner.

A polarized relay device, indicated generally at |99C` in Fig. 14 isprovided for controlling the direction of rotation ofthe azimuth motor 4| when the manually operable azimuth control slider |61 of the potentiometer |66 is adjusted to cause adjustment of the azimuth ring 26. Current is induced, by the primary 86 of the main power transformer in the center tap secondary 89 of the main transformer, the outer or end terminals of the secondary 89 being connected by conductors 200 and 20| to the plates 202 and 203 of a vacuum tube 204, having a -grid 205 or connected grids 205-205 connected by conductm` 266 to the plate |92 of the preceding amplifier tube |84a, a condenser 201 being interposed in the conductor 206. The center tap 09h of the secondary 89 of the main power transformer is connected by a conductor 208 through a ground connection to the adjacent terminals of a pair of relay magnets 209 and 2|0. The cathodes 2|| and 2|2 of the vacuum 204 are connected by conductors 2|4 and 2|5 to the outer orV relatively distant ends of a second pair of relay magnet coils 2 6 and 2 the adjacent ends of the magnets 2|6 and 2|1 being `connected by conductors 2 I8 and 2 I9 to the outer ends of the magnets 209.- and 2| 0, respectively. The magnets 2|6 and 2|] are'disposed with their axes in alignment. The axes of thev magnets 209 and 2|0 arealso disposed in alignment. Relay contacts or armatures 2 8a and 2 9a are pivotally supported, respectively, ends of the magnets 2 |6 and 2|`| and the magnets 209 and2| 0.

The azimuth motor 4I is preferably of the permanent magnet field type, current having a D. C. potential is supplied to the armature windings of the motor through conductors 220 and 22| from a suitable D. C; source indicated at 22|b. Spaced relay contacts 222, 223 and 224, 225 are provided respectively at opposite sides of the relay contact blades 2 8a and 2 I 9a, with cross-over connectors 226 and 221 extending respectively between contacts 223 and 224, and between contacts 222 and 225, as shown in the drawings, the cross-over connectors 226 and 221 being connected intermediate their ends respectively to the conductors 22| and 220, and through the conductors 220a and 22|a to the azimuth motor 4|. A direct current power source, or battery 22B, supplies a suitable negative grid bias to the grids 205 and 205 of the tube 204 through a conductor 229 connected to the negative terminal of the battery 228, a suitable fixed resistor 230 being interposed in the conductor 229. The cathodes 2| and 2|2 of the tube 204 are connected to the positive. terminal of the battery 228 by conductors 23| and 232 having fixed condensers 233 and 234 therein.

The 115 volt, four hundred cycle A. C. which is connected to the primary 86 of the multistage power transformer is preferably from an electrical inverter type of power source, and preferably two independently operable -inverters are employed in mappingairplanes, with suitable switch means, so that either inverter may be interchangeably used as the A. C. electrical power source.k The inverter power source and switching arrangement therefor form no part of the present invention and are not disclosed in detail inthe drawings orv specifically described.

In actual practice it may also bek desirable to between they adjacent 18y employ two or more stabilized camera mounts. These mounts are placed one behind the other, preferably on the longitudinal center of the mapping aircraft. Each of the mounts is similar to the stabilized mount just described and functions independently, although it is contemplated that one mount could function as a master leveling unit and the other mount or mounts would be electrically connected through bridge circuits, controlled by potentiometers, adjusted by tilting and azimuth movements of the master unit, follow-up movement controlling potentiometers being provided and operated by the slave units to balance the aforesaid bridge circuits when the slave units are shifted to their positions as determined by the positions of the azimuth ring 26 of the master leveling unit.

In aerialmapping, in which pictures are takenY in succession from an airplane iiying a known course over territory to be photographed it is, of course, desirable that the exact position of the aircraft be determined with accuracy from certain triangulation points at the time each picture is taken. This can be accomplished by the use of radar, or shoran position determining systems with the radar or shoran stations located at known triangulation positions. An equally important requirementis that of maintaining the photographic axis of the camera vertical at all times. The improved leveling apparatus described above accomplishes this latter condition with a minimum degree of error.

In order to initially adjust the camera support to dispose the camera axis in true vertical position it isfirst preferable to have the main frame |2 as level as conveniently possible, although the apparatus may be calibrated with the main frame out of [level to a considerable degree. The camera supporting azimuth ring 26 may now be leveled', after which it is only necessary to adjust the kcamera on the azimuth ring to level position in which the photographic axis ofthe camera is vertical andthe focal plane of the film is horizontal. This is accomplished by adjusting the jack screws 34 and 34h and tightening the securing bolts 35 while observingA the positions of the bubbles of the two transverse spirit levels 84 and 85. The azimuth ring 26 may be-leveled by adjusting the position of the light beam projection and photocellunit 44 in the two transverse planes. The level position of the azimuth ring may bedetermined in any convenient manner such as by placing a spirit level across the top of the azimuth ring in both pitch and roll planes. Loosening of the clamping screws 54 and adjustment of the set screws 59 shift the photocell unit in the roll plane relative to the azimuth ring 26 and' if the leveling mechanism is in operation the azimuthring 26 will be correspondingly adjusted, after which the clamping screws 54' may be tightened to secure the photocell unit 44 in rigid relation to the azimuth ring in the roll plane. In adjustingthe azimuth ring in pitch the securingbolt 52 is loosened and the screw 5| is then adjusted to shift the photocell unit until the level position of the azimuth ring is determined in the pitch plane. When the camera is mounted on the azimuth ring with' the leveling apparatus in operation` the adjustments of the fore and aftfjack screwsy 34 and the'offset jack screw 34a while observing the positionofthe'bubbles in the spirit levels'64 and 85 within the lower portion of the camera case constitute means for disposing the camera axis in true vertical position or disposing the focal plane of the camera parallel to the top of the azimuth ring 26. Any subsequent leveling corrections of the camera C may be made by an adjustment of the photocell unit 44 about one or both of its adjustment axes. The leveling adjustment may be made at the factory, or at the installation depot, if desired, before the apparatus is installed in the aircraft.

Operation When an aircraft is flown across a terrain to be photographed, With the leveling apparatus installed therein and in operation, with a mapping camera C mounted on the azimuth ring 26, the camera axis will be automatically maintained in a vertical or stabilized position at all times regardless of a considerable degree tilt of the aircraft in the pitch and roll planes and even during relatively short periods of acceleration of the aircraft and turns. The gyroscope rotor 62, being of the free or balanced type and the erection of the spin axis of the rotor to a vertical reference position by a positive and comparative slow erection means, utilizing only the frictional precessing torque on the rotor which is caused by the frictional driving engagement between the small ball-shaped extremity on the upper end of the depending pendulous or motor driven shaft and the internal surface of the inverted V-shaped supporting recess in the underside of the gyroscope rotor 62, average accelerations over a comparative long time period will be integrated to maintain the rotor spin axis at or extremely close to the true vertical at all times, except during extreme prolonged turns and accelerations.

Since the rotor spin axis is vertical the re- I- ilecting surface of the optical flat or mirror 65 rotates in a precise horizontal plane. When the azim uth ring 26 is level the flat rectangular beams of light from the projection lamps 61 and 61a will strike the surface of the revolving mirror 65 at its center in transverse intersecting planes and the light beams will be reflected upwardly, substantially at 45 angles of incidence, each beam passing equally through the adjacent edge portions of one pair of the cylindric lens elements 13. 13 or 13a. 73a any light passing through each of the cylindric lenses being concentrated on tbe elongated cathode element of the photoelectric cell which is located at the focus of the cylindric lenses. cathode elements of each pair of the photoelectric cells 69-10 or G9a-10a determines the leveling adjustments of the camera supporting platform 2S. When the focal plane` of the camera on the azimuth ring 26 and the reflecting surface 65 on the rotor 62 are parallel to each other, each pair of photocells receives an eoual amount of light and the photocells become equally conductive, equal electric potential being im pressed 011 the grids of each of the tubes 9S, IMI associated with the phasing transformers and H2, causing equal current flow through the two primaries |03, |04 and H0, of each of the phasing transformers |05 and H2. Since the secondaries |06 and II3 of each of the phasing transformers I 05 and ||2 are connected in series to the intermediate amplifying stage H9, and are wound in opposition to each other, the phase of the current at the same respective ends of each of the secondaries |06 and ||3 is opposite and the electric potentials are equal, the current in the circuit to the intermediate amplifying stage I I9 is therefore zero. The battery or D. C. current source |28, through the resistor |29. supplies a comparative voltage to the grid which in The relative illumination of the turn controls a comparative voltage of fixed polarity that is impressed on the primary |32 of the transformer |32. Although the electric outputs from the secondaries |06 and |13 are in opposition and equal, causing zero current flow in the circuit to the intermediate amplifying stages, as indicated above, the battery |22 supplies a grid bias to the grid 0f the tube I 9 making this tube partially conductive. rEhe lpotential of the grid |20 is pulsating in the phase and with the amplitude, resulting out of the co-operation of the transformer secondaries llo` and ||3 depending upon which of the photocells is more conductive.

In the event that the camera C and the azimuth ring 26 are tilted the photocell light beam projection unit 44 carried thereby is also tilted, the gyroscope rotor 62 remaining fixed in space displays the light beam in the plane of tilt. Since the light beam normally passes equally through the adjacent edge portions of each of the cylindrie lens members 13, 13 or 13a, 13a, and the beam is wide and thin, a minimum vertical displacement of the light beam relative to the cylindric lenses causes a maximum variation in the amount of light which is centered on the two photocells in the plane of tilt and light displacement. Depending upon the direction of tilt of the azimuth ring 26 relative to the plane of the reflecting surface of the mirror 65 one of the vacuum tubes $9 or |00 will be more conducting while the other tube will become less conductive, and as a result, an alternating current varying in phase and amplitude will be impressed on the grid |20 of the tube IIS in the intermediate amplifying stage H9, depending upon which photocell receives more light. The A. C. impressed on the grid H3 is amplified by the rst stage of the tube ||9l and the ampliiied potential is impressed on the grid |30 of the second stage of the same tube by the conductor |34 and the condenser |35. The second stage serves as a single drive stage supplying the primary |32 of the transformer |32, which is an input transformer for the push pull amplifier stage including the tubes |46 and |41, exciting the primary |54 of the output transformer |55 inducing current in the secondary winding |60 directly connected through the phase adapting condenser |64 to the control coil |6| of the two phase A. C. motor I8.

As the eddy current motor I8 or 2l' rotates the D. C. dynamo I8@ or 21e, driven thereby, causes an opposing D. C. potential to be impressed on the grids 91 or 98 or 91a or 98a of the vacuum tubes 99 or |09 or 99a or |0041. As the speed of the eddy current motor I8 or the eddy current motor 2'| increases an opposing electrical current is built up, causing an electric dampening effect on the current controlled by the tubes 99, |00 or 98a, I00a, so that when the azimuth ring 26 and the camera axis approach their plumb positions and the light beam on the photocell control unit 44 effects a change in the grid potential on the tubes 99 and |00 or 99a, I00a, the electrical potential coming from the D. C. generator Ia or 2`|c and connected to the grids 91, 98 or 91a, 98a in opposition to the photocell potential impressed on the grids, causes a reduced A. C. amplitude to be supplied to the eddy current motors. As the speeds of the eddy current motors I8 or 21 are reduced the D. C. outputs from the electrical generators |8a or 21e are comparatively reduced, and the electrical dampening effects are correspondingly reduced. This elecemacs? `tric dampening control of the eddy current levelingmotors I8 and 21, by the current fgen# verated'bythe D. C. generatorsla and 21c Which are driven by'the eddy 'current motors, produces arapid leveling adjustment of the azimuth ring 26, but at the same time, `the-dampening effect described, together with the low inertia of the rotating parts ofthe eddy current motors, affords a quick deceleration of the eddy vcurrent motors when the current is reduced or cut off without the -usual hunting effect. During small or iinaljleveling 'adjustments bythe eddy current motors, the speed-is vsomewhat Areduced and the dampening eiTect is correspondingly reduced so that in the nal levelingadjustment, orduring very 'small adjustments, the -dampening control effect `is somewhat negligible. The efiiciency of the dampening effectis adjustable by `varying the resistance `of the resistor l I1.

Inithe event of cross windstheamount vof drift is'determined in the usual manner and the air- Vcraft'ight axis is shiftedto maintain the lground trackfin the desired-direction during the mapping operation. The azimuth control potentiometer |86 is then-adjusted to the drift angle -between 'the airplane heading and the ground track. The adjustment of the potentiometer slider I61'unbalances the bridge circuit tothe primary v|19 ofthe transformer |80 and in turn an kelectrical A. C. potential is induced in thesecondary IBI `of `the transformer |80 yand in the electronic 4arnplifyingstage |84. This adjusts `the position 'of ythe polarized -relay contact arms 2'I8a `and '2W-9a which Yconnect the D.-C. azimuth motor 4I tot-he D. ACL source, the polarity ofthe D. C. po-

tential inthe motor circuitfbeing determined 'by ythe direction -inwhich the slider |61 is'adjusted.

As :the D. C.motor 4I rotates,`the Worm on shaft 40, meshing with the azimuth supportgearseg- 'ment is adj usted and thebalancing-potentiom- 'eter'slider Ilythroughthe gear train 43a is correspondingly adj usteduntil ythe output circuit |82, |88 'from the transformer secondary I8| is again balanced andthe relay `contacts 2I8a., 2I9afare opened. 'Theazimuth motor-4I stops and the ro- :tative adiustmentof the'azimuth -ring 25 is discontinued with the longitudinal axis of the film or direction'of film travelin the-cameraparallel to the ground track of the mapping aircraft.

`In the event of an extreme tilt of the mapping s 'aircraft in either of vthe pitch Aor the roll planes, the leveling mechanism vwill be simultaneously operated in an attempt to .maintain the camera supporting platform level and the camera lens axis verticalyhowevergthe relative [controlled `by that Itube is cut off, causing the Aeddy current motor which'is employed'to tilt the camera supporting azimuth ring 26 in that plane (of excessive tilt) to stop. vFurther tilting of the `main frame I2 causes the camera supporting platform 26 and main frame `|2 4to tilt together Aas a unit. When the tilt of the mainframe is Areduced.suiicientlyto permit .the limit switches V8I,"`8I or Bla, 8Ia to open, and disconnect thel bucking coils |31 or |38 from the circuitsthe leveling action will again be operative.

`When it is desired to employ a plurality of the camera leveling device for simultaneouslyitaking a plurality of pictures, additional leveling devices are employed which may be adjusted by convenient bridge circuits to control the operation of ltheleveling motors in the added leveling devices in the manner described in connection with the azimuth motor control. Each of the leveling motors I8 and 21 and the azimuth lmotor drive shaft 4U of the iirst or master leveling control unit would be geared to adjust a potentiometer slider for unbalancing the respective circuits to the leveling motors I8 and 21 and the azimuth motors `II of the other or slave leveling units, and the drive shafts With the leveling motors and the azimuth motor of the slave units would, through suitable gearing, actuate follow-up or balancing potentiometer sliders |16 to balance the bridgeeircuits afore-said when the azimuth rings 26 of the slave leveling units were adjusted to the identical position of the azimuth ring of the master unit.

As before set forth the wiring diagramin Fig. 14 discloses a simplied wiring circuit, employing ya simple electronic amplifier stage but it is to be understood that multistage amplication maybe employed in the construction and Vcarrying out of the present invention without departing from the Spirit rof the invention as definedby the appended claims, also the present illustrated disclosure 'is for purposes of illustration and that this invention includes all rmodifications and equivalents which fall within the scope of the appended claims.

We claim:

1. Inafollow-up system for sensitiveelements, the combination with an A. C. source, sensitive andiollow-up elements, ,ampliiier means and reu versiblemotor means connected to the amplifier 4means voutput for driving the Afollovv-up element',

of alight beamdirectingmeans ,on said sensitive element, photoelectric devices on the follow-up element adapted to intercept a variable amount of light from the light beam directing means as the .relative position of the elements varies, ,a pair of grid-controlled vacuumtubes governed by said photoelectric devices, phase shifting means responsive to the varying relative resistance ofesaid photoelectric devices controlled by the relative amount `of light received by the photo-,electric `devices for oppositely shifting the phase on ,the inputof the ampliermeans to drive the motor .means kin ran `opposite direction, and direct currentrgenerator means driven by ,said reversible motor means having an electrical output connected to at least one of the grids of .the vacuum tubes and operable by said reversible motor means to generate current oi a preden terminecl potential relative to predetermined speeds ofthe reversible motor means to vary :the grid potential of said connected vacuum'tube in rpredetermined'ratio to the rats and direction of yrotation ofthe reversible motor means.

2. In afollow-up system for-sensitive elements,

Athe combination with relatively movable sensitive and follow-up elements and reversible two-phase plified and electrically` Vconnected to the other phase'of the eddy 'current lmotor vmeans in -a ninety degree out-of-phase relation to selectively rotate the motor means in one direction or the other, at predetermined rates, depending upon the relative plate current of the tubesmeans for supplying a plate current to said tubes, a source of light on said follow-up element, a photoelectric device on said follow-up elements having at least two spaced cathodes, means operable by the sensitive element for varying the relative amount of light received by each of the cathodes according to the relative angular displacement between the sensitive and the followup elements, fixed grid bias means between each of said cathodes and the grids of the tubes to predetermine the plate circuit of said tubes to control the phase and potential in the connected phase circuit to the eddy current motor, to selectively drive the eddy current motor in one direction or the other at predetermined speeds in a predetermined ratio to the variation phase and potential in the connected phase of the eddy current motor, and a D. C. generator driven by the eddy current motor having an electrical output connected to the grid and cathode of at least one of the vacuum tubes to vary the bias current potential on the grid of said tube relative to the potential impressed thereon by the connected cathode of the photoelectric device in a predetermined ratio to the rate and direction of rotation of the eddy current motor.

3. In an attitude determininCr device, a support, a mirror adjustably carried thereby, vertical reference means for stabilizing the plane of a mirror in a predetermined plane relative to predetermined changes in attitude of support relative to the vertical reference means, light beam projecting means on the support for projecting two inclined substantially fiat wide beams of light onto the surface of the mirror in transverse intersecting planes, two pairs of photoelectric cell devices disposed on the support, the photoelectric cells of each pair being disposed in juxtaposed side-by-side relation, one photoelectric cell device being disposed on each side of the median plane of each of the intersecting light beams to selectively receive equal amounts of light from the light beam projected in that plane and reflected by the mirror surface, when the support is disposed in a predetermined reference attitude position and to selectively receive unequal amounts of light reflected by the mirror surface when the support is tilted in that plane relative to the mirror surface and vertical reference means due to change in attitude between the support and the mirror, electrically operable rotary means selectively controlled by said photoelectric cell devices operable in one direction in predetermined ratio to light received by one of said photoelectric cells at one side of said median light plane and operable in the opposite direction by the light received by the photocell at the opposite side Of said median light plane.

4. In an attitude determining device, a supporting member subject to tilting movements, a rotary mirror tiltably carried by the supporting member, vertical reference means carried by the supporting members for maintaining the mirror reflecting surface in a horizontal plane during changes in attitude of the supporting member relative to the vertical reference means, light beam projection means on the supporting member for projecting flat beams of light onto the surface of the mirror in inclined transverse planes intersecting at substantially the rotary axis of the mirror, a pair of elongated cylindric lenses 24 disposed on the supporting member in parallel juxtaposed relation at opposite sides of the angle of instance of each of the light beams reflected from the mirror with the juxtaposed portions of the lenses in the median plane of the light beam when the mirror is in horizontal reference position relative to the vertical reference means and the support is level, for concentrating the reflected light rays received by each of the cylindric lenses at the focal line of the lenses, a pair of photoelectric cell circuit controlling means for each light beam mounted on the supporting member, each having an elongated cathode element disposed at the focal line of each of the lenses to receive substantially all the light projected through the cylindric lenses, electronic amplifying means having an electronically amplified output circuit selectively controlled by each pair of photoelectric cell devices, .reversible electric motor means in said output circuits for moving said supporting member in each of said transverse intersecting planes, including electronic amplifying means connected to each of the aforesaid photocell output circuits for amplifying the output circuit controlled by each pair of photoelectric cell devices, an electrical output circuit connected respectively to the reversible motor means and the last mentioned amplifying means for selectively energizing the electrical motor means to rotate the same in opposite directions and at predetermined rates at a. predetermined ratio to the polarity and amplitude of the electrical potential received from each of said last mentioned electronic amplifying means.

5. In a camera leveling device, a support subject to tilting displacement, a` platform gilmbaled thereon for supporting a mapping camera with its photographic axis in a vertical position, a pendulous motor gimbaled on the platform` with a drive shaft axis disposed vertically, a free gyroscope rotor tiltably mounted on said drive shaft having a mirror xed thereon in a plane transverse to the rotary axis of the gyroscope rotor, erecting mean-s between the drive shaft and the rotor for erecting the axis of the rotor to the vertical reference position of the motor drive shaft axis, light beam projecting means on the platform for projecting two inclined flat wide beams of light onto the reflecting surface of the mirror in transverse intersecting vertical planes, two pairs of side-by-side photoelectric cell devices fixed relative to the platform in said transverse intersecting planes, constructed and arranged to selectively receive substantially equal amounts of the light reflected by said mirror in the re spective transverse planes when the gyro rotor axis is vertical and the platform is horizontal, and to selectively receive predetermined different amounts of the light reflected by the mirror when the platform is tilted relative to the vertical axis of the gyroscope rotor, an electrical output circuit controlled by each photoelectric cell device, electronic amplifying means disposed on the platform adjacent each of the photoelectric cell devices and connected to the photoelectric cell controlled circuits for amplifying the current controlled by said photoelectric cell devices, secondary electronic amplifying means connected to the first amplifying means for amplifying the current from the first electrical amplifying means, electric motor means disposed on the platform gimbal means and electrically connected to the secondary amplifying means, an operating connection between the electric motor means and the platform and between the electric motor means and the support for adjusting the position ofthe platform relative to the support in a predetermined ratio to the amplitude ofthe electrical output of the second current amplifying means and in a predetermined direction tothe polarity of said electrical output, electric potential generatmeans for impressing the output potential of theelectric potential generatingmeans onto the grid of at least one tube of the rst mentioned electronic amplifying meansl to vary the plate current lpotential in predetermined ratio to the rate and direction ofrotation ofthe electric motor means. 6. In a camera leveling mount for mapping cameras, a rigid supporting frame adapted to be carried by an aircraft, outer and inner gimbals carried by said frame for tilting displacement in transverse intersecting vertical planes, two phase eddy current motor leveling means operable between the rigid frame and the outer gimbal member, a second two phase eddy current motor leveling means operable between the inner and outer gimbals, a supporting platform rotatably journalled on said inner gimbal for rotation in azimuth, electric motor means operable between the inner gimbal'and the platform for-adjusting said supporting platform in azimuth, a photoelectric cell and light beam leveling unit adjustably secured on said platform including a gravity erected Areference member having a vertical reference axis, a mirror fixed on rsaid reference member having a nat reflecting surface disposed in a plane perpendicular to the vertical reference axis with a center concentric to the vertical reference axis, light beam projecting means fixed on said leveling unit for projecting two inclined rflat beams of light onto the reiiecting surface of the mirror Yin transverse intersecting planes at similar angles of incidence intersecting at the vertical reference axis when theplatform is level and the reference axis is vertical, a pair of positive elongated parallel cylindric lens elements disposed in juxtaposed edge-to-edge relation in a plane perpendicular to the median plane of each ofthereflected flat light beams with the abutting Ylongitudinal edge portions of the lens members lying inthe median plane when the platform is level and the vertical reference axis is vertical, said -cylindric .lens elements each having a vfocal line .an elongated cathode plate disposed substantially on the focal line of each cylindric lens element and an anode, electrical control circuits connected to the anode and cathode plates of each pair of the photoelectric cells, a source of alternating ,currenty apair of dual plate and grid'triode vacuumtubes lfor each pair of the photoelectric cells,`

each tube having grid means connected to .the cathode plate of one of the photoelectric fcell members, transformer means for each pair of photocell members having a primary connected to the alternating current source and a center tap secondary, a circuit connection between the center tap of said secondary and the cathodes of the said pair of associated vacuum tubes, a grid .bias source connected between the grids afore- 26 said and the cathodes of each of the vacuun' tubes in each pair, a reversibleD. C. generator'for each pair of vacuum tubes, each generator having an electrical output circuit connected between the cathode and grid of at least one of the asso`A` ciated vacuum tubes of each pair, for varying the grid potential on the grid of the connected vacuum tube, with respect'to the grid potential on the grid of the other associated vacuum 'tube in' a` predetermined ratio4 to the rate and direction 'of rotation of the D. C. generator, driving meansv between each of the leveling motors and the'D. C'. generator means associated with the photocells' and vacuum* tubes that are made electrically more orless'conductive by the light beam which is dis-v placed by operation'of thatv leveling motor, aV pair of phasing transformers for eachlight beam'projecting means, each transformer having a pair of opposing lprimaries and a secondary,l the opposing primaries of each of the phasing trans)- formers being connected at one end tothe plates of one of the'vacuum tubes'for each of the photoelectric cells controlled bythat light beam, an electrical output circuitsaid phasing transformer secondaries being connected in said output circuit in seriesin opposition to each other, connec- 'tionsbetween the other ends of the primaries of primary of the second centertap secondary transformer andthe output of said comparative voltage amplifier means, a second output voltage circuit from 'said vacuum tube amplifier means including means for impressing a predetermined grid potential onthe grid of the last mentioned tube amplifier means to control the polarity and "potential of the current in said comparative voltage output circuit depending upon 'which of the photcelectric cell members are conductive and Vthe relative extent of the 'conductivity of theco'n- 'nected phctoelectric cell members, means "for impressing the comparative voltage potential and polarity, controlled by the grid of the tube, on lthe voltage 'in the first-mentioned comparative voltage circuit, dual 'vacuum tube amplifying means, an output circuitfor each of the center tap secondaries of the said center tap secondary transformer means comprising a center tap primary transformer vhaving the center tap connected to the cathodes of the'last mentioned dual vacuum tube amplifying means, the opposite ends of the primary being connected to the plates `of the last dual vacuum tube amplifying means, a secondary 4for the .last mentioned center tap primary transformer connected to one phase `of the twophased eddy current leveling motor means, separate predetermined phase supplying transformer means having a primary connected to the A. C. electrical supply and secondary means connecting the last secondaryto the second phase of the eddy current motor means, and means for shifting the phase in one of said eddy current motor phase circuits to dispose the phases in .out-of-phase relation.

7. uIn` a stabilizing apparatus of 'the class described, a main frame subjectto tilting displace- 

